Means and method for extinguishing an arc spot



April 10, 1945.

c. G. SMITH MEANS AND METHOD FOR EXTINGUISHING AN ARC SPOT Filed Dec. 11, 1941 fi-fiERwuomc EMISSION CURRENT L RETURN CURRENT-I k P sMA I M LA I I CATHODEI 2/ DARK SPAQE SURFACE LAYER OF IGH ELECTRON ARC TEMPERATURE DIFFERENCE EXTENT CURRENT 'D HG PooL.

F'\ G. E,

LoAD l6 INVENTOR. CHARLES C. SMITH,

Patented Apr. 10, 1945 r orric MEANS AND METHOD FOR EXTINGUISHING AN ARC SPOT Charles G. Smith, Medford, Mass., assignor to Raythecn Manufacturing Company,--Newton, Mass, a corporation of Delaware Application December-11, 1941, Serial No. 422,562

9 Claims.

This invention relates to means and method for extinguishing an are spot, particularly on a pool type cathode of an electrical space discharge Heretofore attemptsto control currents of relv atively high values. by extinguishing the flow of current in an arc discharge tub have been unsuccessful because of the large amount of power necessary and the unreliability of theresultant operation. I believe that the failure of such prior attempts has been due largely to the fact that a tenable theory of operation of the are spot had not been developed, and consequently the proposed methods of current stoppage control were not properly adapted to the phenomena existent in the arc discharge. i

The primary object of this invention is to devise areliable means for, extinguishing an are spot, and thus control the fiow of current in an arc discharge tube.

This andother objects of my invention will be best understood from the following descrip-' tion of an exemplification thereof, reference being had to the accompanying drawing, wherein:

Fig.1 is a diagram illustrating certain principles of the theory of operation of my invention; and i Fig. 2 is a diagrammatic representation of a system with which my invention may be practised.

In. accordance with my invention I have been able to overcome the previous diiiiculties, and have devised means whereby relatively large arc currents have been reliably extinguished by the use of surprisingly small amounts of controlled power and in remarkably short intervals of time. I believe that these resultsare due to my new analysis of the are spot conditions and to my development of a tenable theory of the operation of such arc spots.

In accordance withmy present understanding, the theory of the operating mechanisms of an are spot is substantially. as follows. In Fig.1 is represented a small cross-section through the surface of va mercury pool cathode on which an are spot extending between the two vertical broken lines may be considered as in operation. Immediately adjacentthe surface of the cathode above the are spot is the cathode darkspace which is populated mainly by positive ions. The thickness of thisspace is ordinarily considered as being ofthe order of 10-. centimeters. Beyond that space is located a plasma of electrons and ionsin substantially equal numbers. The potentialv fall through the cathode dark space "is order of nine voltsand probably less.

termed the cathode drop, and may be of the The temperature of the electrons in the plasma may be of the order of 30,000 K., which means that the electrons in the plasma are in thermal agitation and are travelling at various speeds in all directions. The average speed of the electrons expressed in equivalent volts may be of theorder of about four'volts. Since this is an average value, a great many electrons in the plasma will have a component of velocity toward the cathode in excess of the cathode drop. These electrons, therefore, have suiiicient velocity to overcome the cathode drop, whereby they pass through the cathode dark-space and fall upon the cathode surface, and deliver considerable energythereto. This energy consists of the kinetic energy of the electrons, and also the condensation energy there of. ,The body of the cathode pool, including the surface layer, may be considered as consisting of a lattice of relatively heavy atoms within which is located an atmosphere of relatively free 'conduction electrons. Although the lattice of a liqtively free conduction electrons within the atomic lattice of the surface of the mercury pool. As a result of this transfer of energy, the temperature of the electrons in a very thin layer at the surface of the mercury pool is raised to a relatively high value which may be of the order of 4500 K. This electronic, temperature is, vof course, not in equilibrium with the temperature of the atomic lattice which is probably of the order of about 200 C. However, the rate of energy transferred to the electronic atmosphere by the impinging electrons is suificiently high to maintain this temperature differential between the electronic atmosphere and the atomic lattice in spite of energy transferred to the atomic lattice from the lectronic atmosphere.

The relatively high temperature of the electronic atmosphere within the surface of the mercury pool gives rise to a large emissionv which may be considered in the nature of thermionic In order to differentiate between the emission.

ordinary type of thermionic emission, in which the electronic atmosphere and the atomic lattice are in quilibrium, I may refer to this new type of emission as p-thermionic emission. When the current carried by an arc is ordinarily measured, the quantity observed is the difference between the rate at which the electrons leave the cathode surface to enter the space above it and the rate at which electrons return to the cathode surface from the plasma, as described above. In Fig. 1 the c-thermionic emission current L is represented by the solid arrows, leaving the surface of the mercury pool, the returning electron current I is represented by the dotted arrows reentering the surface of the cathode, and the difference electronic current D is represented by the solid arrow in the body of the mercury pool.

In absence of some means to maintain the relatively high temperature of the electronic atmos-' phere at the surface of the mercury pool, the heat would be rapidly conducted away from the surin which:

0=temperature of the conduction electrons in the surface layer of the liquid cathodes; and =work function of mercury.

In Equation 2 I have neglected the energy transfer due to positive ions. However, this is such a small factor that it does not substantially affect our present considerations. We also have determined that the c-thermionic emission current is equal to the sum of the difference current and the return current, namely L=D+I (Equation 3) From a consideration of the energy equation as well as from various other factors, I have determined that the current I may be of the order of 2000 or .3000 amperes per square centimeter under certain conditions, while the current D is of the face layer by the conduction electrons, and transferred to lower regions of the mercury pool. It is well known that if a temperature gradient exists in a conducting body and an electronic current is passed through that body along the temperature gradient, there is a transfer of heat due to the current. This is known as the Thomson effect. The direction in which this transfer takes place ma be different in different conducting materials. However, in the case of mercury under the conditions here existing, the trans fer of heat is in the direction in which the electronic current flows. If we look at Fig. 1 we see that the electronic current in the body of the mercury pool is the difference current D which is in a direction to transfer heat to the surface of the mercury pool from the regions below said surface. volved is sufiiciently great so that the heat transfer by conduction through the conduction electrons is compensated for by the Thomson effect. I believe that the elevated temperature of the conduction electrons is confined to a thin surface layer on the mercury having the thickness of the order of a few hundred atomic diameters, approximately 10* centimeters.

We can derive some valuable information by equating the energy input to the cathode surface by the impinging electrons and the energy loss therefrom by the evaporating or ,e-thermionic emission electrons. First we can formulate the equation for the return electronic current:

We can then give the energy belance equation as follows:

- ;kT+eI 10 L 10 +-gike (Equation 2 This transfer under the conditions in-' order of 3000 or 4000 amperes per square centimeter. Thus L under these conditions would be of the order of 5000 to 7000 amperes.

Having established the above relationships, I conceived that if at any time conditions were changed so that the temperature of the conduction electrons in the are spot was reduced substantially, the ,B-thermionic emission would be decreased to such an extent as to be incapable of maintaining the arc spot. In accordance with my invention I successfully accomplished the extinguishment of the arc spot by suddenly increasing the difference current D in an interval of time sufficiently short so that the state of excitation of the plasma remained substantially constant. From Equations 2 and 3 we see that an increase in D under these conditions would produce a direct decrease in I, so that the rate at which energy is supplied to the are spot is decreased. Thi in turn produces a decrease in the temperature of the conduction electrons in the surface of the cathode, and a consequent decrease in the s-thermionic emission. Such decrease produces, of course, a higher cathode voltage drop, which in turn tends to still further decrease the ability of electrons to return to the cathode, and thus still further decreases I. This cumulative effect in an interval of time, which may be of the order of 10" of a second is sufficient to extinguish the are spot. Depending upon the rate at which equilibrium conditions can establish themselves in the plasma, this interval of time may be considerably longer or shorter. The criterion which should exist in each case is that the rate of change of the current D should be greater than that at which equilibrium conditions can establish themselves in the plasma. Another manner in which extinguishment may occur is to interrupt or reverse the current D which causes the Thomson effect to maintain the high temperature gradient at the surface of the cathode. If the current D is interrupted or reversed likewise for a very short period of time of the above order of magnitude, heat is conducted from the surface layer to the conduction electrons below it at such a rapid rate as to lower the temperature of the conduction electrons to such an extent that the thermionic emission and consequently the are adapted to connect the condenser an insulator, separated from. thcsuriaceioi the mercury pool bymeans .of an insulatinslayer thereonin contactwith said surface. Current; is supplied to the tube |"'from apair of terminals 5-8 which, for example, are-adapted.- to beachnectedto a suitablesource of direct current. This source of. current is connected through a load in series with the tube I so that the current in the load I may be controlled. High frequency choke coils 8-9 may be included in the circuit for purposes to be :described below. In order to supply igniting impulses to the electrode 4, an igniting transformerv Ill may be provided having a secondary winding connected between the igniting electrode 4 and the cathode 2. The transformer I may alsobe provided with a primary winding l2 suppliedwith impulses of igniting current from a condenser I3 adapted to be charged from a suitable source of current, such as a battery I4, connected in series with a re-. sistance l5 across said condenser. A-switch H5 is l3 when charged to the primary winding 2 in order to supply an igniting impulse to the igniting electrode 4. When such an igniting impulse is supplied and an arc spot is started on the cathode 2,

current will flow through the tube and the load 1.

principles of my invention as outlined .above may be utilized. The energy for the rapid change of the current through the tube may be supplied from a condenser H which is adapted to be charged with a predetermined polarity from a suitable source of current, such as a battery |8, connected in series with a resistance lfi'across said condenser. In order that the polarity to which the condenser I! is to be charged may be reversed if desired, a reversing switch I1 is interposed between the battery l8 and the condenser The charged condenser I! may be connected through a switch 20 to-a coil 2|. In series with this circuit may be included a resistance 2| wherever it is desired that the discharge from the condenser l1 shallbe'damped or nonoscillatory. In this case the resistance 2| is of sufficient magnitude to produce the desired damping. Inductively coupled to the coil 2| is a coil 22 connected in series with a condenser 23 between the cathode 2 and the anode 3. likewise in series with this circuit may be included a small resistance 24.

In the preferred mode of operation of the system shown in Fig. 2, the condenser I! is charged to polarities as indicated, so that upon closure of the switch 20, a discharge current flows in the direction of the arrow in coil 2| and an induced current flows in the direction of the corresponding arrow in coil 22. It will be seen that this induced current is in a direction to tend to cause the current through the tube I to increase. This current impulse is of a very sharply rising characteristic so as to tend to increase the current through the tube I within the necessary short time interval; as described above, in order to cause the arc spot to be extinguished. In a particular instance I have used the following values for the elements of this extinguishing circuit. The condenser I! was 2 10- farads charged to about 500 volts, the 'coils 2| and 22 were each a single turn of copper four inches in diameter, the resistance 2| was five ohms, the resistance 24 was substantially zero, and the condenser 23 was 5 10 iarads. Under these conditions currents Here of about 20.. amperes could; be readily extin: wished in theftube l within the short time interval as described above. I believe that the peak lncreasein current duringthis :pulse should be of the order of magnitude ofthat of the preceding arc current art-somewhat less. The chokes 8 and 9 previously; referred toare inserted for the purpose of preventing the sharpl rising pulse of current supplied from the condenser H from bypassing thetube lrthrough the load I.

Upon reversal of the switch it will be seen that the condenser l1 will be charged to the opposite potential, and upon closure of the switch 20 a pulse of current tending to reduce and reverse'thevcurrent through the tube I will be inducedin the coil 22. I have found that with the same constants as previously used, currents of the order of twenty amperes can likewise be readily extinguished in the tube in the short time interval described above. I have found, however,-that extinguishing an are spot by reduction'or 'reversaloi the current ordinarily requires a greater amount of energy stored in the condenser than in the case of extinguishment by a sudden increase in the current flowing through the tube.

For most purposesI prefer to have the condenser 23 of substantially greater value than that described in the particular example which I have given. In connection with the extinguishing circuit in the particular embodiment which I have shown, the condenser 23 serves to prevent the direct current from'short-circuiting across the tube I. However, by making this condenser of substantial size, the reliability with which the starting electrode initiates an arc spot is greatly increased, as more fully described and claimed in the copending application of Wilcox P. Over beck, Serial No. 271,679,'fi1ed May 4, 1939, for an improvement in Arc tube systems. We see. therefore, that the circuit connected between the cathode 2 and the anode 3 may be made to serve the double function of facilitating the initiation of the are spot, and also serving to extinguish the are spot.

Of course it is to be understood that this inven tion is not limited to the particular details of construction, magnitude, and the like, as described above, as many equivalents will suggest themselves to those skilled in the art. For example, the principles of my invention can be applied to polyphase rectifier tubes or to any type of tube in which an are spot may be extinguished for purposes of controlling a current. It is believed that the principles which I have enunciated are so broadly new that an extremely wide and varied number of utilizations thereof will suggest themselves to those skilled in the art. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is cla med is:

1. The method of extinguishing an are spot on a localized are spot type of cathode which comprises suddenly chang ng the current flow to said cathode, said change being carried out at a rate so rapid that said are spot is extinguished in a period of time of the order of 10* second or less after the initiation of said change.

2. The method of extingu shing an are spot on a localized are spot type of cathode which comprises increasing the flow of current to said are spot at such a rapid rate as to cause said spot to be extinguished, said change being carried out at a rate so rapid that said arc spot is extinguished in a period of time of the order of 10" second' or less' after the initiation of said change.

3. The method of extinguishing an are spot on a localized are spot type of cathode which comprises reducing the flow of current to said are spot at such a rapid rate as to cause said spot to be extinguished, said change being carried out at a rate so rapid that said are spot is extinguished in a period of time of the order of 10- second or less after the initiation of said change. 4. In combination wtih a d scharge tube of the type comprising a localized are spot type cathode and a cooperating electrode, means for establishing and maintaining an are spot onsaid cathode, and means for suddenly changing the flow of current to said are spot at a rate so rapid that said are spot is extingu'shed in a period of time of the order of 10* second or less'after the initiation of said change.

5. In combination with a discharge tube of the type comprising a localized are spot type cathode and a cooperating electrodameans for establishing and maintaining an are spot on said cathode, and means for suddenly increasing the-flow of current to said arc spot at a rate so rapid that said arc spot is extinguished ina'period of time of the order of 10" second or less after the initiation of said change. 6. In combination with a discharge tube of the type comprising a localized are spot' type cathode and a cooperating electrode, means for establishing and maintaining an are spot on said cathode, electrical energy storage reservoir means.

and means for connecting said reservoir means to feed energy between said cathode and cooperating electrode ina direction to increase the flow of current to said are spot, the impedance of that portion of the system leading from said reservoir mean to said cathode and cooperating electrode being sufliciently small to current surges andsaid reservoir means being of sufiicient'size to cause said change in current flow to occur at a. sufficiently great rate to extinguish said are spotin a period of time of the order of 10- second or less after the initiation of said change; I

'7. In combination with a discharge tube of the type comprising a localized are spot type cathode and a cooperating electrode, means for establishing and-maintaining an are spot on said cathode, electrical energy storage reservoir means, and means for connecting said reservoir means to feed energy between said cathode and cooperatin electrode in a direction to reduce the flow of current to said are spot, the impedance of that portion of the system leading from said reservoir means to said cathode and cooperating electrode being sufiiciently small to current surges and said reservoir means being of suflicient size to cause said change in current flow to occur at a sufficiently great rate to extinguish said are spot in a period of time of the order of 10- second or less after the initiation of said change.

8. In combination with a discharge tube of the type comprising a localized are spot type cathode and a cooperating electrode, means for establishing and maintaining an are spot on said cathode, condenser means, and means for connecting said condenser means to feed energy between said cathode and cooperating electrode in a direction to increase the flow of current to said arc spot, the impedance of that portion of the system leading from said condenser means to said cathode and cooperating electrode being sufficiently small to current surges and said condenser means being of sufiicient size to cause said change in current flow to occur at a sufiiciently great rate to extinguish said are spot in a period of time of the order of 10- second or less after the initiation of said change.

9. In combination with a discharge tube of the type comprising a localized are spot type cathode and a cooperating electrode, mean for establishing and maintaining an are spot on said cathode, condenser means, and mean for connecting said condenser means to feed energy between said cathode and cooperating electrode in a direction to reduce the flow of current to said are spot, the impedance of that portion of the system leading from said condenser means to said cathode and cooperating electrode being sufciently small to current surges and said condenser means being of sufiicient size to cause said change in current flow to occur at 'a sufliciently great rate to extinguish said are spot in a period of time of the order of 10- second or less after the initiation of said change.

CHARLES G. SMITH. 

